Digital to analog convertors (DAC) are known to be used in almost every type of audio and/or visual equipment to convert processed digital signals into analog signals which may then be render audible, if the analog signals are audio signals, or render visible if the analog signals are video signals. The audio/visual equipment produces the processed digital signals by receiving an input analog signal (representative of audio signals or video signals) and converts such analog signals into digital signals. In the digital domain, the audio/visual equipment filters, mixes, compresses/decompresses, encrypts/decrypts, and/or modulates/demodulates the digital signals to produce the processed digital signals.
One commonly used DAC is a Sigma-Delta DAC. A Sigma-Delta DAC includes an interpolation filter, a digital noise shaper (Sigma-Delta modulator) and an analog reconstruction filter. The interpolation filter receives a multi-bit digital word having a first sampling frequency and converts the digital word into a multi-bit word having a second sampling frequency, where the second sampling frequency is greater than the first. Such frequency adjustments are referred to as interpolation.
The multi-bit words having the second sampling frequency are then converted into single bit words having the second sampling frequency by the digital noise shaper. This conversion uses single bit quantization, but introduces quantization noise. Most of the energy of the quantization noise is above the audio frequency range.
Typical analog reconstruction filters may be a low pass filter constructed using switch-capacitor techniques and/or resistor-capacitor (RC) circuits. While switch-capacitor low pass filters may readily be implemented on CMOS integrated circuits, they have a non-linear phase response and signal dependent charge response which causes distortion which restricts their applicability. The RC circuits provide a wide dynamic ranges, but require precise component matching to achieve the desired results.
U.S. Pat. No. 5,323,157, issued to Ledzius et al, discloses a Sigma-Delta digital-to-analog converter that includes an analog reconstruction circuit that avoids the problems mentioned above. In the Ledzius patent, the sigma delta DAC includes the interpolation filter, the sigma delta modulator, and a finite impulse response (FIR) filter which functions as the analog reconstruction circuit. As is known, an FIR filter may be configured, by scaling FIR coefficients, to provide low pass filtering of digital signals and provide a coarse analog output. The coarse analog output may be further filtered by a single pole low pass filter.
In the Ledzius patent, the FIR coefficients are provided by tuned current sources coupled to a summing node. A current source provides a current to the summing node when an associated digital bit is a logic "1" and does not provide a current when the associated digital bit is a logic "0". While this implementation overcomes the drawbacks of the RC circuits and the switched capacitor circuits, it requires the current sources to be matched, which is difficult using CMOS technology. As is known, FIR coefficients have a wide range of values which results in a large disparity between the smallest coefficient and the largest coefficient. Due to this large disparity, it is very difficult to match the current sources using CMOS technology. If the current sources are not matched, the FIR filters performance is diminished, especially in stop band attenuation. To improve the FIR filters performance, additional taps, or FIR coefficients are needed, thereby increasing the size and complexity of the FIR filter.
Therefore, a need exists for a method and apparatus that implements a digital to analog convertor that uses oversampling techniques but does not have the limitations of the prior art described above.